A new study has found that children, who have at least one smoker parent, have 5.5 times higher levels of cotinine, a byproduct of nicotine, in their urine.

The study, published online ahead of print in Archives of Disease in Childhood, showed that having a smoker mother had the prime independent effect on cotinine in the urine - quadrupling it. Having a smoking father doubled the amount of cotinine, one of chemicals produced when the body breaks down nicotine from inhaled smoke to get rid of it.

The study was led by researchers from the University of Leicester Medical School and was done in collaboration with Warwick University.

Sleeping with parents and lower temperature rooms were also linked to increased amounts of cotinine.

Cotinine was measured in 104 urine samples taken from 12-week old infants. Seventy one of the babies had at least one parent that smoked and the parents of the other 33 were non-smokers.

"Babies affected by smoke tend to come from poorer homes, which may have smaller rooms and inadequate heating. Higher cotinine levels in colder times of year may be a reflection of the other key factors which influence exposure to passive smoking, such as poorer ventilation or a greater tendency for parents to smoke indoors in winter," the authors said.

Sleeping with a parent is a know risk factor for cot death and the authors suggest that one reason for this could be breathing of, or contact with clothing or other objects contaminated with, smoke particles during sleep.

Nearly 40 percent of under-fives are believed to be exposed to tobacco smoke at home, and smoke may be responsible for up to 6,000 deaths per year in the US alone in young children.

"Babies and children are routinely exposed to cigarette smoking by their caretakers in their homes, without the legislative protection available to adults in public places," the authors said.

But they admitted that there are realistic difficulties in preventing smoking in private homes because it relies on parents or caretakers being educated about the harmful effects of passive smoking on their children and then acting on that knowledge.